Recently it has become evident that spoofing poses a significant threat to GNSS receivers. Hence, spoofing detection and mitigation has become an active area of research. The authentic GNSS signal sourced from a satellite Space Vehicle (SV) is very weak at the location of the terrestrial receiver and is therefore vulnerable to hostile jamming based on narrowband noise transmission. As the GNSS frequency band is known to the jammer, its effectiveness is easily optimized by confining its radiation to within the relatively narrow GNSS signal band. The transmit power requirements of a jammer placed several kilometers away from the GNSS receiver is modest with several Watts Equivalent Isotropically Radiated Power (EIRP) being sufficient to deny the GNSS receiver of any reliable pseudo-range estimates. There are several means of mitigating such noise jammers, namely:
Increased Processing Gain Based on Using Longer Coherent Integration Times
The processing gain of the GNSS spread spectrum receiver is given as the product of the bandwidth of the complex baseband signal and the coherent integration interval which can in principle be increased arbitrarily. However, in dynamic platform scenarios, a minimum update rate must be maintained limiting the coherent integration interval. Considering a high dynamic case where a 1 ms update rate is required, based on a GPS C/A signal with bandwidth of about 1 MHz, then the processing gain is limited to about 30 dB. Hence a jammer power of only −100 dBm at the GPS receiver output will result in a signal to jammer ratio of approximately 0 dB which is insufficient for robust signal detection.
Adaptive Null Steering
A GNSS receiver equipped with multiple antennas can provide null steering in the direction of the jammer. Adaptive processing that tracks the bearing of the jammer can be implemented. The depth of the null is a function of the platform dynamics of the jammer and GNSS receiver. In static scenarios 40 to 50 dB of nulling is possible with just two antennas however; very precise phasing of the two antennas is required. A further disadvantage of this method is that a minimum of two spatially separated antennas will be required. Note that as the GNSS signals are mutually orthogonal, adaptive processing can be applied to each SV signal independently. Also, typically, the jammer will originate from a single bearing and hence a minimal array of only two antennas is sufficient to null out the jammer.
GNSS Diversity
Recently more sources of GNSS signals have become available in different frequency bands with the receiver can exploit by limiting observables to signals that are not jammed. However, the jammer can obviously counter this by simultaneously radiating noise in the various relevant GNSS bands.
Navigation Diversity
The user of the GNSS receiver may have alternate means of navigation which will be used as an alternate to the compromised GNSS outputs.
Physically Disabling Jammer
Ultimately the jammer can be easily located and physically disabled.
While noise jamming of the GNSS receiver is a threat, the user is easily aware of its existence and characteristics. The worst case is that GNSS based navigation is denied. A more significant jamming threat that is currently emerging is that of the spoofing jammer where bogus signals are transmitted from the jammer that emulates authentic GNSS signals. This is done with multiple SV signals in a coordinated fashion to synthesize a plausible navigation solution to the GNSS receiver. The objective of the jammer is then to cause the navigation solution as generated by the GNSS receiver to drift away from the true position. The drift is carefully orchestrated such that the GNSS receiver is unaware that it is being spoofed. The consequence of a drifting navigation solution believed to be authentic is generally more dire to the GNSS user than a GNSS receiver disabled by jamming that the user is aware of. Fortunately, spoofing is often detectable as the bogus SV signals generated by the jammer move too quickly or too erratically which is detectable by a tracking filter. Furthermore, to be effective, the bogus navigation solution synthesized by the jammer has to sweep through the true solution currently tracked by the GNSS receiver and to capture it similar to the classical range gate pull off methods applied to radar jamming. The GNSS receiver tracking filter can further incorporate multiple ancillary sensor signals in addition to the GNSS signals to verify the plausibility of the computed navigation solution.
An exploitable weakness of the spoofing jammer is that for practical deployment reasons, the spoofing signals generally come from a common transmitter source. Hence a single jamming antenna sources the spoofing signals simultaneously. This results in a means of possible discrimination between the real and bogus GNSS signals as the authentic GNSS signals will emanate from known bearings distributed across the hemisphere. Furthermore, the bearing of the jammer as seen from the GNSS receiver will be different than the bearing to any of the tracked SV's. This immediately sets up some opportunities for the GNSS receiver to reject the spoofing jamming signals. Some of these opportunities are as follows:                (i) Processing can be built into the GNSS receiver that estimates the bearing of each of the SV signals. Note that the relative bearings of the GNSS signals are sufficient in this case as the bogus GNSS signals will all have a common bearing while the authentic GNSS signals will always be at different bearings. If the GNSS bearings are not consistent with the expected distribution then an alarm can be generated indicating the possibility of spoofing signals.        (ii) Unobstructed SV signals will reach the GNSS receiver with a signal strength that is known within a small range. If the received signal is significantly stronger than expected then spoofing can be suspected. If the spoofing signal is too weak it will not capture the GNSS receiver tracking.        
If the GNSS receiver has multiple antennas and if the position of the antennas is such that there is an unobstructed line of sight (LOS) to the SV's then there are possibilities of:                Detection of the spoofing based on the common bearing of the received GNSS signals.        Eliminating all the jammer signals simultaneously by appropriate combining of the receiver antennas to form a pattern null coincident with the jammer bearing.        
Unfortunately the above will not be an option if the jammer signal or SV signals are subjected to spatial multipath fading. In this case, the jammer and individual SV signals will come in from several bearings simultaneously. Another problem is if the GNSS receiver is constrained by the form factor of a small handset device such that an antenna array is not an option. As the carrier wavelength of GNSS signals is on the order of 18 to 25 cm, at most two antennas can be considered for the handset receiver. Such a handset receiver with two antennas can be considered as an interferometer that has some ability for relative signal bearing estimation as well as nulling at specific bearings. However, such an antenna pair is not well represented by independent isotropic field sampling nodes but will be significantly coupled and strongly influenced by the arbitrary orientation that the user imposes. Hence the handset antenna is poorly suited for discrimination of the spoofing signal based on bearing. Furthermore, the handheld receiver is typically used in areas of multipath or foliage attenuation and therefore the SV signal bearing and strength are random with significant variation.
There is therefore a need for methods and devices which can be used to detect spoofing or signals originating from an inauthentic source.